Conventional virgin aluminum production typically involves the reduction of alumina which has been dissolved in a cryolite-containing electrolyte. The reduction is carried out in a Hall-Heroult cell ("Hall cell") containing a carbon anode and a carbon cathode which also serves as a container for the electrolyte. When current is run through the electrolyte, liquid aluminum is deposited at the cathode while gaseous oxygen is produced at the anode.
The sidewalls of the Hall cell are typically made of a porous, heat conductive material based on carbon or silicon carbide. However, since it is well known in the art that the cryolite-containing electrolyte aggressively attacks these sidewalls, the sidewalls are designed to be only about 3-6 inches thick so as to provide enough heat loss out of the Hall cell to allow the formation of a frozen layer of cryolite on the surface of the sidewall, thereby preventing further cryolite infiltration and degradation of the sidewall.
Although the frozen cryolite layer successfully protects the sidewalls from cryolite penetration, it does so at the cost of significant heat loss. Accordingly, modern efficiency concerns have driven newer Hall cell designs to contain more heat insulation in the sidewalls. However, since these designs having significant thermal insulation also prevent significant heat loss, cryolite will not freeze against its sidewalls. Therefore, the initial concerns about cryolite penetration and sidewall degradation have reappeared.
U.S. Pat. No. 4,592,820 ("the '820 patent") attempts to provide both thermal efficiency and sidewall protection from cryolite penetration. The '820 patent teaches replacing the porous, heat conductive sidewall with a two-layer sidewall comprising:
a) a first layer made of a conventional insulating material provided in sufficient thickness to assure that cryolite will not freeze on the sidewall, and PA1 b) a lining made of a ceramic material resistant to attack by the cell electrolyte (cryolite) and molten aluminum. PA1 a) providing an aluminum reduction cell comprising a cathode, an anode and a sidewall, the sidewall having a thickness and comprising: PA1 b) contacting the lining with an electrolyte comprising at least 60% cryolite and having a temperature of between 650.degree. C. and 1100.degree. C., and PA1 c) providing an electric current from the cathode to the anode through the electrolyte, thereby producing aluminum at the cathode, PA1 a) the insulating material is provided in sufficient thickness to assure that cryolite will not freeze anywhere but the top edge of the sidewall, and PA1 b) the lining consists essentially of a ceramic material having a density of at least 95% of theoretical density and at least closed porosity, the ceramic material selected from the group consisting of silicon carbide, silicon nitride and boron carbide, PA1 i) means for maintaining the molten fluoride electrolyte at a temperature of about 960.degree. C., and PA1 ii) a sidewall comprising an insulating material and a lining, wherein: PA1 a) the insulating material is provided in sufficient thickness to assure that cryolite will not freeze anywhere on the lining, and PA1 b) the lining is made of a ceramic material resistant to attack by cryolite and molten aluminum, PA1 a) the insulating material is provided in sufficient thickness to assure that cryolite will not freeze anywhere on the lining, and PA1 b) the lining is made of a ceramic material resistant to attack by cryolite and molten aluminum,
See column 2, lines 30-43 of the '820 patent. The '820 patent further discloses that preferred linings are made of Group IVb, Vb or VIb refractory metal carbides, borides or nitrides, oxynitrides and especially titanium diboride and teaches these selected ceramic materials can be used as either fabricated tiles or as coatings on sidewalls such as alumina or silicon carbide. See column 2, lines 44-47 and column 4, lines 24-32.
Although the '820 patent provides a cryolite-resistant aluminum reduction cell having improved heat efficiency, it nonetheless can be improved upon. For example, the disclosed linings suffer from high cost and limited availability.
Moreover, the preferred lining of the '820 patent, titanium diboride, is not only very expensive, it also possesses marginal oxidation resistance and is electrically conductive in operation.
In addition, the preferred Hall cell of the '820 patent produces a solid cryolite layer in the electrolyte zone adjacent the top edge of the sidewall to protect the ceramic material against aerial oxidation. This top layer may be developed by either capping the sidewall with carbon and reducing its backing insulation, or by positioning a steel pipe carrying cool air adjacent the top edge of the sidewall. Although these measures improve cryolite resistance, they also reduce the heat efficiency of the cell.
U.S. Pat. No. 4,865,701 ("Beck") discloses an aluminum production cell having cooling tubes provided within the insulating layer of its sidewall.
U.S. Pat. No. 2,971,899 ("Hannick") discloses a cell for electroplating aluminum from a solution containing about 20% cryolite. U.S. Pat. No. 2,915,442 ("Lewis") discloses an aluminum production cell wherein a frozen crust appears on the sidewall. U.S. Pat. No. 3,256,173 ("Schmitt") discloses an aluminum production cell having a lining of silicon carbide, coke and pitch. U.S. Pat. No. 3,428,545 ("Johnson") discloses an aluminum production cell having a carbon lining backed by refractory particles including silicon nitride.
Accordingly, there is a need for an improved Hall Cell.